Polytrimethylene terephthalate (PTT) is primarily a linear aromatic polyester which can be prepared from the esterification and condensation polymerization of 1,3-propanediol (PDO) and terephthalic acid (TPA) or 1,3-propanediol and alkyl diesters of terephthalic acid. Polytrimethylene terephthalate is currently used in carpet fibre and textile fibre applications. For such commercial applications, it is desired to produce polytrimethylene terephthalate which has good spinning behaviour, dyeability and colour stability. It is also desired to produce polytrimethylene terephthalate which has low tendency to generate acrolein when the polymer is heated in air, as it commonly is during downstream processing such as spinning into fibres. It is known that this instability can be controlled with additives such as hindered phenols such as described in WO98/23662.
The tendency of polytrimethylene terephthalate to produce acrolein when heated and other final properties of the PTT polymer are related to the amount of dipropylene glycol (DPG) monomer units produced in the polymer during polymerization.
Dipropylene glycol (DPG) is used herein to mean the residual unit [—OCH2CH2CH2OCH2CH2CH2O—] in the polymer chain. The following formula shows how the DPG units are copolymerized into the polymer to form a random copolyester (and how some end groups, such as allyl, carboxyl, methyl ester, may be incorporated):
comprising units A and B connected randomly to each other by ester bonds and wherein E is an endgroup attached to the diol end and consisting of either H or the residuum of a hindered phenol (defined herein), E′ is an endgroup attached to the carboxyl end and consisting of a PDO [—OCH2CH2CH2OH], a DPG [—OCH2CH2CH2OCH2CH2CH2OH], an allyl [—OCH2CH═CH2] group, a hydroxy [—OH] or, in the case when alkyl diesters of terephthalic acid are used, an alkoxy [—OR] group, such as methoxy [—OCH3], and where the average (or overall) molar proportion of DPG comonomer units to the total diol units, including endgroups, is the ratio of the sum of y+E′ (where E′ is the DPG endgroups only) to the sum of x+y+E′ (where E′ is the diol endgroups, including allyl, but excluding the hydroxy and alkoxy endgroups) and is in the range of from about 0.001 to about 0.05. The average degree of polymerization is x+y and is greater than 50 and preferably greater than 80, typically greater than 100. The average number of endgroups E+E′ is 2 or less relative to x+y.
The amount of dipropylene glycol comonomer units present in polytrimethylene terephthalate has a noticeable effect on the final properties of the polymer. The level of DPG comonomer units present in the PTT polymer affects the polymerisation and the crystallinity of the polymer and thus the melting point. Therefore the level of DPG comonomer units present in the final PTT polymer will have an effect on the properties of the PTT during further processing, such as spinning and/or dyeing of PTT fibers. If the level of DPG comonomer units present in the PTT polymer is too low, this results in poor dyeability of the product and if the level is too high, this can result in excessive levels of acrolein being produced upon heating of the PTT in air. Therefore, accurate control of the amount of dipropylene glycol comonomer units in the final polytrimethylene terephthalate product is desired. Unfortunately, however, the linear isomer dipropylene glycol is not a commercially available product, and therefore the level of dipropylene glycol comonomer units present in polytrimethylene terephthalate cannot be controlled by separate addition of DPG. The level of DPG comonomer units in the final PTT polymer therefore has to be controlled by the process of production of PTT.
Various methods are known in the art for the production of PTT. Each of these methods give varying levels of DPG comonomer units in the final PTT polymer.
PTT made by the solid state polymerization process described in U.S. Pat. No. 6,403,762 (which comprises esterification of 1,3-propanediol (PDO) and terephthalic acid (TPA) to form an esterification product followed by polycondensation of the esterification product to form a prepolymer which is solid state polymerized to produce a polymer with the desired intrinsic viscosity), comprises a significant amount (from about 1.8 to about 3.0 mole %) of DPG comonomer units. The PTT polymer produced by this process has good properties for subsequent processing of the polymer, such as dyeability of the fiber, but has a tendency to produce acrolein when heated in air.
PTT made by the so-called “all melt process”, such as that described in U.S. Pat. No. 6,509,438 (which comprises esterification of 1,3-propanediol (PDO) and terephthalic acid (TPA) followed by a prepolymerization and a final polycondensation step to obtain a PTT polymer of desired intrinsic viscosity), comprises lower amounts of DPG comonomer units (from about 0.6 to about 2.0 mole %) than PTT made by the above mentioned solid state polymerization process. The PTT polymer produced by the “all melt process” has a lower tendency to generate acrolein on heating in air, but dyeing and spinning properties of the fibre can be affected.
PTT made by processes involving transesterification of 1,3-propanediol and dimethyl terephthalate (DMT), such as described in JP-A-51-142097 or U.S. Pat. No. 5,340,909, although oxidatively stable, comprises very low amounts of DPG comonomer units (from about 0.05 to about 0.5 mole %), which although produce low levels of acrolein on heating in air, can cause difficulties with the dyeing and the spinning properties of the PTT fibre.
Since customers have specific requirements for PTT and require polymers with consistent properties, it would be highly desirable to be able control the amount of dipropylene glycol comonomer units present in polytrimethylene terephthalate. The ability to control the amount of DPG comonomer units in the manufacture of PTT would not only ensure that the polymer produced would have a consistent amount of DPG comonomer units, and thus consistent properties, but would also enable the tailoring of the DPG comonomer unit content of the PTT, and thus the final properties of the PTT, to meet various customer requirements.